1. Field of the Invention
This invention relates to extrudable organosiloxane compositions. More particularly, this invention relates to a unique class of extrudable organosiloxane compositions that cure by the platinum-catalyzed reaction of alkenyl radicals with silicon-bonded hydrogen atoms to form elastomeric materials. The cure rate of the present compositions is considerably faster than prior art compositions wherein the ethylenically unsaturated radicals are vinyl.
2. Description of the Prior Art
Curable organosiloxane compositions containing at least one polydiorganosiloxane with at least two silicon-bonded alkenyl radicals, an organohydrogensiloxane or organic peroxide curing agent and a silica filler are well known in the art pertaining to silicone elastomers. It is also known to improve the physical properties, particularly tensile and tear strengths, of cured elastomers prepared from these curable compositions by using two or more vinyl-containing polydiorganosiloxanes of different molecular weights or a single vinyl-containing polydiorganosiloxane having a bimodal molecular weight distribution.
British patent no. 1,269,007 that issued to Wada and Ito on Mar. 29, 1972 teaches peroxide curable compositions containing two diorganosiloxanes polymers (1 and 2) each of which has at least 3000 diorganosiloxane units per molecule and a third polydiorganosiloxane containing from 10 to 1000 siloxane units per molecule. Three different vinyl-containing diorganosiloxane polymers are used to achieve high tear strength in the cured elastomer. Polymer 1 contains either no vinyl radicals or from 0.02 to 0.3 mole percent of vinyl radicals at either terminal or non-terminal positions in the molecule; polymer 2 contains from 5 to 20 mole percent of vinyl-containing siloxane units; and polymer 3 contains from 5 to 90 percent of vinyl-containing siloxane units. The tensile strength of the cured elastomers are about 9800 kPa and the maximum tear strength value reported is 45 kilonewtons per meter (kN/m). The curable compositions would have the consistency of a gum.
U.S. Pat. No. 3,671,480, which issued on June 20, 1972 to the same inventors named on the aforementioned British patent, teaches organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction to yield elastomers exhibiting high values of tear strength. These compositions contain two different vinyl-substituted polydiorganosiloxanes. The first of these polymers contains from 0.02 to 0.2 mole percent of vinyl-substituted siloxane units and an average of at least 3000 siloxane units per molecule, which is equivalent to a gum-type polydimethylsiloxane having a viscosity of at least 1000 Pa.s. The vinyl radicals of this polymer can be located on either terminal or non-terminal repeating units. In all of the exemplified compositions the aforementioned first polymer contains non-terminal vinyl radicals. The second polymer contains an average of at least 100 siloxane units per molecule and at least 2 mole percent of vinyl-substituted siloxane units.
U.S. Pat. No. 4,162,243, which issued to Lee et al. on July 24, 1979, discloses organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. The molecular weight distribution of the vinyl-containing polydiorganosiloxane described by Lee et al. yield cured 9elastomers exhibiting higher durometer hardness values using less silica filler, with a resultant decrease in the viscosity of the curable composition. This is achieved by the presence in the curable composition of Lee et al. of (1) a silica filler with vinyl-containing organosiloxane groups bonded to its surface and (2) a triorganosiloxy-terminated polydimethylsiloxane composition wherein the triorganosiloxy terminal groups are defined as dimethylvinylsiloxy or phenylmethylvinylsiloxy and the composition exhibits a specified molecular weight distribution and dispersity index. The required molecular weight distribution can be achieved using a single polydimethylsiloxane or a mixture of two or more polydimethylsiloxanes.
The triorganosiloxy terminated polydimethylsiloxanes present in the compositions of the aforementioned Lee et al. patent by definition exclude the presence of ethylenically unsaturated hydrocarbon radicals in any non-terminal repeating unit.
U.S. Pat. No. 4,753,978, which issued on June 28, 1988 to Jary Jensen, one of the present applicants, teaches using two vinyl-containing polymers of different molecular weights and vinyl content in a liquid curable organosiloxane composition to prepare elastomers exhibiting a unique combination of high tensile and tear strengths without adversely affecting other desireable properties of the elastomer. This is achieved by using a high molecular weight vinyl-containing polydiorganosiloxane having vinyl radicals only at the terminal positions and a second, lower molecular weight polydiorganosiloxane wherein from 1 to 5 percent of the non-terminal silicon atoms contain a vinyl radical.
The use of higher alkenyl radicals to increase the cure rate of organosiloxane compositions curable by a platinum-catalyzed hydrosilation reaction is taught in the prior art. U.S. Pat. No. 4.609.574, which issued to Keryk et al. on Sept. 2, 1986 discloses silicone release coatings wherein at least 50 percent, preferably 100 percent of the alkenyl radicals that react with silicon-bonded hydrogen atoms during the curing reaction contain 6 or more carbon atoms and terminal unsaturation. The advantage of using the higher molecular weight alkenyl radicals relative to lower molecular weight radicals such as vinyl is a faster and more complete reaction during curing of the composition.
A preference for higher alkenyl radicals in organosiloxane compositions that cure by a radiation-initiated reaction between alkenyl radicals and mercapto groups is disclosed in U.S. Pat. No. 4,596,720, that issued to Keryk and Lo on June 24, 1986.
U.S. Pat. No. 4,611,042, which issued to S. Rivers-Farrell et al. on Sept. 9, 1986 teaches resinous copolymeric siloxanes consisting essentially of trimethylsiloxy, alkenyldimethylsiloxy and SiO.sub.2 units, where the alkenyl radicals are terminally unsaturated and contain at least six carbon atoms. The presence of these alkenyl radicals in place of vinyl radicals is considered responsible for the faster cure rate of the compositions.
The present inventors investigated the effect on cure rate of substituting 5-hexenyl for vinyl radicals on the two types of vinyl-containing polydiorganosiloxanes present in curable compositions of the type described in the aforementioned U.S. Pat. No. 4,753,978. They discovered that the type of ethylenically unsaturated radical present on the polydiorganosiloxane containing non-terminal unsaturated radicals had a considerably greater effect on cure rate than the type of ethylenically unsaturated radical present on the polydiorganosiloxane containing these radicals only at the terminal positions, even when the concentration of alkenyl radicals contributed by each of the two types of polydiorganosiloxanes was equal.